Fuel systems typically employ multiple closed-nozzle fuel injectors to inject high pressure fuel into the combustion chambers of an engine. Each of these fuel injectors includes a nozzle assembly having a cylindrical bore with a nozzle supply passageway and a nozzle outlet. The efficiency of the nozzle outlet or orifice is a measure of how effectively the energy stored in the fuel as pressure is converted into kinetic energy. The greater the kinetic energy, the more the fuel is broken apart (atomized), improving combustion completeness and lowering soot. High-efficiency (HE) nozzles, i.e., those with the highest orifice efficiency, are desirable for emissions.
Unfortunately, HE nozzles also have a greater propensity to exhibit coking, or injector spray hole fouling, which is the deposition of coked fuel layers on the orifice wall (internal) and on the outside surface of the nozzle tip (external). The flow rate of a coked nozzle is reduced because of the added restriction to the flow. As rated injection pressures of new injection systems increase to further provide emission benefits, it has become increasingly difficult to design HE nozzles without coking.
Coking is when the byproducts of combustion accumulate on or near the injector nozzle openings. As the deposits build up, they can clog the injector nozzle orifices and adversely affect the performance of the fuel injectors. This can lead to reduced fuel economy and can increase the amount of pollutants released into the atmosphere through exhaust.
To date, the problem of coking has been addressed by engine manufactures seeking nozzle designs that avoid flow rate losses, deemed unacceptable when the loss is more than about three percent. One method for maintaining high nozzle efficiency without coking has been to minimize the spray hole aspect ratio (L/D)—the ratio of the spray hole length (L) to the spray hole exit diameter (D). The ability to further decrease spray hole length (L) is constrained by the allowable stresses in the nozzle metal as injection pressure increases. The ability to further increase spray hole exit diameter (D) is constrained by the nozzle flow rate and the number of holes that are best for emissions for a given engine application. Other methods, such as increasing spray hole internal roughness or making subtle changes in spray hole geometry, provide only marginal improvements to reduce coking.
The device of the present disclosure is directed to overcoming the problems set forth above, but in a way previously unappreciated by those skilled in the art. The present device provides a unique operation strategy which makes use of HE nozzles and requires few additional components over those currently used in fuel injection systems. The device and methods of the present invention recognize and take advantage of two previously unappreciated facts: (1) flow rate loss due to coking will eventually stabilize after sufficient service time, and (2) good emission performance can be maintained even with coked nozzles.